This application claims the priority benefit of Taiwan application serial no. 91103734, filed Mar. 1, 2002.
1. Field of Invention
The present invention relates to a solder ball attaching process. More particularly, the present invention relates to a process of attaching a solder ball to the bonding pad of a wafer.
2. Description of Related Art
Due to the trend for developing light and compact electronic products, the size of most integrated circuit packages continues to decrease. To reduce the size of integrated circuit (IC) packages, chip scale packages (CSP) are developed. In general, the edge length of a CSP package is roughly 1.2 times the edge length of a silicon chip or the chip/package has an area ratio of about 80% and the pitch between leads is limited to a value under 1 mm. Many types of chip scale packages are now available. However, the most common type is one having the package directly formed on the wafer known also as a wafer level chip scale package (WLCSP).
One major characteristic of a WLCSP is the fabrication of a redistribution circuit on the surface of the chip so that the bonding pads around the periphery of the chip are redistributed as an area array on top of the chip. Hence, the entire surface of the chip can be utilized for accommodating bonding pads, thereby producing a larger pitch between bonding pads to meet the larger distance of separation between contacts on a printed circuit board (PCB).
To connect the chip and the printed circuit board electrically, a solder ball is normally attached to the bonding pad corresponding to the redistribution circuit above the chip surface. Thus, the original bonding pad on the chip surface may connect electrically with the contact on the printed circuit board through the solder ball and the redistribution circuit on the chip. On the other hand, if the original bonding pad on the chip has sufficiently large distance of separation, a redistribution circuit may not be required. In this case, the solder balls may be directly attached to the bonding pads on the chip such that the original bonding pads on the chip are directly connected to the contacts on a printed circuit board via the solder balls. In the following description, the bonding pads on the chip or the wafer may refer to the original bonding pads on the chip or the bonding pad on the chip after circuit redistribution.
A conventional solder ball attaching process involves positioning a solder ball onto a corresponding bonding pad and performing a reflow process so that the solder ball is firmly attached to the bonding pad. At present, two major methods are used for positioning the solder balls, namely, the vacuum transfer method and the gravity transfer method. In the vacuum transfer method, suction on a suction head is used to pick up a solder ball from a solder ball holder and then transfer it to a corresponding bonding pad location. In the gravity transfer method, a stencil is placed over the wafer surface and then a large number of solder balls is spread out over the stencil. Through the weight of each solder ball and a rocking motion produced by a motor, the solder balls are forced into the sieve holes in the stencil and hence appropriately positioned over the bonding pads. However, a layer of flux material is smeared over the bonding pad surface to increase the bonding strength between the solder balls and the bonding pads before the solder balls are positioned and the reflow process is performed.
The equipment for positioning the solder balls using a vacuum transfer method is rather expensive. The stencil for positioning the solder ball in a gravity transfer method must be accurately aligned over the wafer. Moreover, the stencil must change for each new wafer design and the expenses for drilling a large number of sieve holes in a stencil is high. All in all, overall cost for attaching solder balls onto bonding pads is high.
Accordingly, one object of the present invention is to provide a solder ball attaching process for attaching a plurality of solder balls to bonding pads on a wafer. The attaching process costs much less than the conventional vacuum transfer method or the gravity transfer method. Moreover, this invention provides openings having a step structure to facilitate the trapping of solder balls and the positioning of solder balls accurately over the bonding pads within a short time.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a solder ball attaching process for attaching at least one solder ball onto a wafer. The wafer has an active surface, a passivation layer and at least one bonding pad. The passivation layer and the bonding pad are formed on the active surface of the wafer. The passivation layer exposes the bonding pad. The solder ball attaching process includes forming at least one under-ball-metallurgy layer over the bonding pad. Thereafter, a first masking layer is formed on the active surface of the wafer and then patterned to form at least one opening that exposes the under-ball-metallurgy layer. A second masking layer is formed over the first masking layer and then the second masking layer is patterned to form at least one second opening that exposes the under-ball metallurgy layer. The second opening, having an aperture, such as diameter, greater than the first opening, is positioned over the first opening. Thereafter, a solder ball is placed on the second masking layer and allowed to roll gently so that the solder ball drops into the second and the first opening. The lower end of the solder ball is in contact with the upper surface of the under-ball metallurgy layer. A reflow process is conducted so that the solder ball and the under-ball metallurgy layer are bonded together. Finally, the first masking layer and the second masking layer are removed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.